1. Field of the Invention
The disclosed technology relates to multi-channel biopotential signal acquisition systems and, more particularly, to active electrode based systems that comprise enhanced common-mode rejection ratio techniques.
2. Description of the Related Technology
Active electrodes have been employed where electrodes are integrated with amplifiers for the suppression of interference picked up from cables. An ideal active electrode comprises a passive electrode and a pre-amplifier that are co-integrated in the same package which can be placed very close to the skin to extract low-level biopotential signals. In this way, the signal path length between the electrode and the pre-amplifier is minimized, maintain the highest possible input impedance of the amplifier. Furthermore, the output of the active electrode forming a low-impedance node and the interference and motion artifacts obtained by cable movement and electromagnetic fields in the vicinity can both be reduced when compared to a conventional passive electrode interface where a high impedance node between the electrode and the amplifier picks up interference currents.
Common mode (CM) interference is one of the major problems in such active electrode biopotential signal acquisition systems. CM interference at inputs of the active electrode can be converted to a differential mode (DM) error at the outputs of the active electrode pairs due to the voltage gain mismatch between these active electrode pairs. The output error can have significant large amplitude when compared to the amplitude of the biopotential signals. As a result, in multi-channel biopotential acquisition systems, it is necessary to have good common-mode rejection ratio (CMRR) to reject 50 Hz or 60 Hz CM interferences whilst extracting μV level biopotential signals.
State-of-the-art active electrode systems employ voltage buffers to achieve good CMRR between electrode pairs. One such active electrode is described in “A Simple Active Electrode for Power Line Interference Reduction in High Resolution Biopotential Measurements” by M. Fernandez and R. Pallas-Areny, Proc. 18th Annual International Conference, IEEE Engineering in Medicine and Biology Society, vol. 1, pages 97 to 98, 1997. In this active electrode, a voltage buffer is used to facilitate inter-channel gain matching that is necessary to achieve a high CMRR. However, low noise buffers consume significant power and, due to their lack of gain, still require the use of a back-end that is low noise and power hungry to maintain the total integrated noise at acceptable levels.
An amplifier with gain can effectively reduce the power consumption from the back-end processor whilst achieving the same input referred noise of the system. However, gain mismatch between electrode pairs usually limits the CMRR to between 60 dB and 70 dB. One design is described in “A Micro-Power EEG Acquisition SoC with Integrated Feature Extraction Processor for a Chronic Seizure Detection System by N. Verma, A. Shoeb, A. J. Bohorquez et al, IEEE J. Solid-State Circuits, pages 804 to 816, April 2010, where the CMRR is limited to 60 dB by capacitor mismatch.
Common-mode feedback (CMFB) has been found to be a solution for reducing effective CM interferences and improving CMRR between amplifier pairs. Such a method is described in “Highly Sensitive Biomedical Amplifier with CMRR Calibration and DC-Offset Compensation” by W. Galjan, K. M. Hafkemeyer, J. M. Tomasik, F. Wagner, W. H. Krautschneider and D. Schroeder, EUROCON2009. Here, a known CM input is applied during calibration where the output DM error voltage is sensed, digitized and fed back to adjust a resistive digital-to-analogue converter (DAC) so that the voltage gain of the amplifier pairs can be matched. During amplification, amplifiers, for which the gain has been calibrated, amplify the biopotential signals with enhanced CMRR. However, calibration for a multi-channel system disturbs continuous-time monitoring.
A continuous-time CMRR enhancement method employing CMFB is described in “High Quality Recording of Bioelectric Events I: Interference Reduction, Theory and Practice” by C. A. Grimbergen, A. C. Metting Van Rijn and A. Peper, EMBC 1990. A circuit, used in such a method, is known as a driven-right-leg (DRL) circuit. An output CM sensing circuit is used and the output CM voltage is fed back to the patient by way of a reference electrode. The CMFB circuit can reduce the effect of CM gain and therefore improves CMRR.
However, DRL circuits suffer from stability problems and require large power dissipation as the CM signal is fed back through the reference electrode with significant large impedance described in “A Transconductance Driven-Right-Leg Circuit” by E. M. Spinelli et al., IEEE Transaction on Biomedical Engineering, Vol. 46, No. 12, pages 1466 to 1470, December 1999.